Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1017

Rh At the south-west corner of the Lizard peninsula is a mass of talco-micaceous shales, described by De la Beche (p. 29), and separated in his map. As far, however, as I can make out, they only form a zone with some slight lithological peculiarities in the hornblendic schist, into which they seem to pass almost insensibly. These may be well studied in the descent to the shore at Polpeor, and in the base of the cliffs there.

Following the cliffs from Polpeor for about a mile in a straight line, we come to the first junction with the serpentine, at the south end of the narrow strip of sand called Pentreath Beach; a little chine runs almost along the line of junction. To make out the relations of the two rocks here is no easy task; both are extremely decomposed for some yards, and traversed by numerous cracks, which are filled with calcite, often stained red with hæmatite, and project like a network from the weathered ground. The rocks are thus almost brecciated in situ. The difficulty is caused by the close resemblance of the two rocks in their extremest decomposition, so that it is sometimes almost impossible to separate them. After two or three visits and a most minute examination, I think I have succeeded. The cliff on the north bank of the little chine is all serpentine; on the south the headland is all hornblende schist; but after a few yards the serpentine rises from the shore and forms the lower part of the cliff, its boundary curving gradually upwards. Thus there is not really a passage from the one rock to the other here, or a faulted junction, but the serpentine is intrusive. The serpentine then forms the cliffs as far as they can be followed; above them, near the upper end of the beach, and some 50 feet above it, a granite vein breaks in two or three places through the serpentine, which is cracked and altered at the junction; the granite is finely crystalline, chiefly composed of quartz and felspar, with only a little mica; it is of a pinkish grey colour, becoming red and friable in weathering. One mass has carried up some irregular fragments of hornblendic schist. Close by is another mass of hornblendic schist, included in the serpentine, possibly forced into it by the granite; and another included mass may be seen on the beach below. On the hillside, a short distance beyond, is a serpentine-quarry. Two of the varieties of the rock obtained here are very pretty (no. 1 ):—one, of a dull red colour, irregularly mottled with a waxy-looking dull green mineral, with occasional flakes of greenish bronzite, rather hard and irregular in fracture; the other, a dull purplish red, veined with greyish green, the latter generally fringing a thin dark line, like a crack, and forming a sort of polyhedral network. It is probably only the result of decomposition, but produces a very pretty effect. At the north-east angle is a bifurcating granite vein, about